Field of the Invention
This disclosure is related to the field of polymer interlayers for multiple layer glass panels and multiple layer glass panels having at least one polymer interlayer sheet. Specifically, this disclosure is related to the field of polymer interlayers comprising multiple thermoplastic layers which have improved optical and acoustic properties and resist formation of certain optical defects.
Description of Related Art
Multiple layer panels are generally panels comprised of two sheets of a substrate (such as, but not limited to, glass, polyester, polyacrylate, or polycarbonate) with one or more polymer interlayers sandwiched therebetween. The laminated multiple layer glass panels are commonly utilized in architectural window applications and in the windows of motor vehicles and airplanes, and in photovoltaic solar panels. The first two applications are commonly referred to as laminated safety glass. The main function of the interlayer in the laminated safety glass is to absorb energy resulting from impact or force applied to the glass, to keep the layers of glass bonded even when the force is applied and the glass is broken, and to prevent the glass from breaking up into sharp pieces. Additionally, the interlayer may also give the glass a much higher sound insulation rating, reduce UV and/or IR light transmission, and enhance the aesthetic appeal of the associated window. In regard to the photovoltaic applications, the main function of the interlayer is to encapsulate the photovoltaic solar panels which are used to generate and supply electricity in commercial and residential applications.
In order to achieve the certain property and performance characteristics for the glass panel, it has become common practice to utilize multiple layer or multilayered interlayers. As used herein, the terms “multilayer” and “multiple layers” mean an interlayer having more than one layer, and multilayer and multiple layer may be used interchangeably. Multiple layer interlayers typically contain at least one soft layer and at least one stiff layer. Interlayers with one soft “core” layer sandwiched between two more rigid or stiff “skin” layers have been designed with sound insulation properties for the glass panel. Interlayers having the reverse configuration, that is, with one stiff layer sandwiched between two more soft layers have been found to improve the impact performance of the glass panel and can also be designed for sound insulation. Examples of multiple layer interlayers also include the interlayers with at least one “clear” or non-colored layer and at least one colored layer or at least one conventional layer, e.g., non-acoustic layer, and at least one acoustic layer (i.e., a layer have acoustic properties or the ability to provide sound insulation or reduce sound transmission, as further defined below). Other examples of multiple layer interlayers include interlayers with at least two layers with different colors for aesthetic appeal. The colored layer typically contains pigments or dyes or some combination of pigments and dyes.
The layers of the interlayer are generally produced by mixing a polymer resin such as poly(vinyl butyral) with one or more plasticizers and melt processing the mix into a sheet by any applicable process or method known to one of skill in the art, including, but not limited to, extrusion. Multiple layer interlayers can be produced by processes such as co-extrusion or lamination wherein the layers are combined together to form a unitary structure. Other additional ingredients may optionally be added for various other purposes. After the interlayer sheet is formed, it is typically collected and rolled for transportation and storage and for later use in the multiple layer glass panel, as discussed below.
The following offers a simplified description of the manner in which multiple layer glass panels are generally produced in combination with the interlayers. First, at least one polymer interlayer sheet (single or multilayer) is placed between two substrates and any excess interlayer is trimmed from the edges, creating an assembly. It is not uncommon for multiple polymer interlayer sheets or a polymer interlayer sheet with multiple layers (or a combination of both) to be placed within the two substrates creating a multiple layer glass panel with multiple polymer interlayers. Then, air is removed from the assembly by an applicable process or method known to one of skill in the art; e.g., through nip rollers, vacuum bag or another deairing mechanism. Additionally, the interlayer is partially press-bonded to the substrates by any method known to one of ordinary skill in the art. In a last step, in order to form a final unitary structure, this preliminary bonding is rendered more permanent by a high temperature and pressure lamination process, or any other method known to one of ordinary skill in the art such as, but not limited to, autoclaving.
Multilayer interlayers such as a trilayer interlayer having a soft core layer and two stiffer skin layers are commercially available. The stiff skin layers provide handling, processing and mechanical strength of the interlayer; the soft core layer provides acoustic damping properties. One of the problems in the manufacture of multilayer laminate glass panels having multiple layer interlayers is the presence of mottle in the final unitary structure. The term “mottle” refers to an objectionable visual defect in the final unitary structure, namely the appearance of uneven spots, a form of optical distortion. Stated differently, mottle is a measure of the graininess or texture formed from the optical effect of reflecting non-uniform distorted interfaces of the inner polymer interlayer or polymer interlayers.
In multiple layer interlayers having at least one soft layer and at least one stiff layer, the mottle is caused by small scale surface variations at the interfaces between the layers wherein the individual layers (or the soft and stiff layers) have different refractive indices. When the polymer interlayer is produced, surface roughness is formed at the utmost surface of the polymer interlayer through melt fracture or embossing or both. The surface roughness enables and improves removal of air during laminating of polymer interlayers to produce multiple layer glass panels, and helps to prevent the blocking of the polymer interlayers during storage. However, such surface roughness will also cause the development of small scale surface variations at the interfaces between layers of the multilayer interlayer to form mottle.
The refractive index of a substance, such as an interlayer, is the measure of the speed of light through the substance with respect to the speed of light in vacuum. If there is a difference between the refractive index of the layers, the result will be that the surface variations at the interface are visible or even more visible due to diffraction of the light at the layer interfaces. Mottle is theoretically possible with any multiple layer interlayer, especially where there is a sufficiently large difference in the refractive indices between the layers and there is some degree of interfacial variation between the layers.
The presence of mottle in the final unitary structure of a multilayer laminate glass panel can be problematic because a certain degree of optical quality is necessary in many (if not most) of the end-use commercial applications of multilayer laminate glass panels (e.g., vehicular, aeronautical and architectural applications). Thus, the creation of multilayer laminate glass panels with commercially acceptable levels of mottle (that is, where the level of mottle is low) is paramount in the art of multiple layer glass panel manufacturing.
To ascertain the level of mottle in a laminate, the severity of the mottle is assessed and categorized by a side-to-side qualitative comparison of the shadowgraph projections for a test laminate with a set of standard laminate shadowgraphs representing a series or scale of mottle values ranging from 1 to 4, with 1 representing a standard of low mottle (i.e., a low number of disruptions) and 4 representing a standard of high mottle (i.e., a high number of disruptions), which is optically objectionable. Based upon a visual interpretation of which standard laminate shadowgraph picture the test shadowgraph projection best corresponds with, the test laminate is then placed into the mottle category of the corresponding standard laminate. Shadowgraph pictures can also be captured by a digital camera and analyzed by digital image analysis tools to give digitalized results or mottle ratings. Digitalized rating is calibrated using the same set of standard laminates representing a series of mottle values ranging from 1 to 4. A xenon arc lamp is used as the light source for shadowgraph projections.
Clarity of the multiple layer panel is another important optical quality. Clarity is determined by measuring the level of haze in the multiple layer panel, as further described below. The level of haze must be very low so that the multiple layer panel is clear. In addition to haze, there are other optical quality defects, such as visible optical defects in the interlayer, that cause light scattering and make the defect visible to the eye that may cause optical distortion in the glass panel as well. Both haze and other visible optical defects are caused by light scattering due to the blending or mixing of materials, such as different polymers or plasticizers, together, or the contamination from such different polymers or plasticizers where there is a sufficiently large difference in the refractive index between the different polymers or plasticizers, the matrix and the contaminants, or both.
Summarized, optical quality defects such as mottle, haze and other visible optical defects are common problems in the field of multiple layer glass panels, particularly those used in applications which require higher levels of optical or visual quality. It is now common to use a multilayer interlayer in order to provide high performance laminates. The use of multilayer interlayers, however, has very often resulted in having optical defect problems, such as mottle. Accordingly, there is a need in the art for the development of a multilayered interlayer that has good optical, mechanical, and acoustic characteristics desirable in a multilayered interlayer. More specifically, there is a need in the art for the development of multilayered interlayers having at least one soft core layer that has good acoustic properties and resists formation of optical defects.